In the auditory system, the tonotopic organization of the basilar membrane means that high-frequency sounds cause maximal vibration at the base and low frequencies at the apex. Which property of the basilar membrane explains this frequency gradient?

• A The basilar membrane is wider and more compliant (less stiff) at the base, resonating with high frequencies
• B The density of inner hair cells is highest at the base, making it sensitive to high-frequency sounds
• C The basilar membrane is narrower and stiffer at the base, resonating with high frequencies; wider and more compliant at the apex, resonating with low frequencies ✓
• D Endocochlear potential is highest at the base, amplifying high-frequency signals

Explanation

The basilar membrane exhibits a gradient: at the basal end (near the stapes), it is narrow (~0.1 mm) and stiff (high resonant frequency — 20,000 Hz); at the apical end, it is wider (~0.5 mm) and compliant/loose (low resonant frequency — 20 Hz). This physical gradient creates the tonotopic map exploited by the cochlear partition. High-frequency sounds maximally displace the stiff base; low-frequency sounds travel further and maximally displace the compliant apex. Hair cell density and endocochlear potential do not create this frequency gradient — the mechanical properties of the membrane itself do.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.